In a normal type of quadrupole mass spectrometer, various kinds of ions created from a sample are introduced into a quadrupole mass filter, which selectively allows only ions having a specific mass-to-charge ratio to pass through it. The selected ions are detected by a detector to obtain an intensity signal corresponding to the amount of ions.
The quadrupole mass filter consists of four rod electrodes arranged parallel to each other around an ion-beam axis, with a voltage composed of a direct-current (DC) voltage and a radio-frequency (RF) voltage (alternating-current voltage) being applied to each of the four rod electrodes. The mass-to-charge ratio of the ions that are allowed to pass through the quadrupole mass filter depends on the RF and DC voltages applied to the rod electrodes. Therefore, it is possible to selectively allow an intended kind of ion to pass through the filter by appropriately setting the RF and DC voltages according to the mass-to-charge ratio of the target ion. Furthermore, when each of the RF and DC voltages applied to the rod electrodes is varied within a predetermined range so that the mass-to-charge ratio of the ion passing through the quadrupole mass filter continuously changes over a predetermined range, a mass spectrum can be created from the signals produced by the detector during this process (the scan measurement).
A detailed description of the voltage applied to the rod electrodes of the quadrupole mass filter is as follows: Among the four rod electrodes, each pair of electrodes facing each other across the ion-beam axis are electrically connected to each other. A voltage U+V·cosΩt is applied to one pair of the electrodes, while a voltage −U−V·cosΩt is applied to the other pair, where ±U and ±V·cosΩt are the DC and RF voltages, respectively. A common DC bias voltage, which may additionally be applied to all the rod electrodes, is disregarded in the present discussion since this voltage does not affect the mass-to-charge ratio of the ion that can pass through the filter. In the aforementioned case of changing the mass-to-charge ratio of the target ion over a predetermined range, the voltage value U of the DC voltage and the amplitude value V of the RF voltage are normally controlled so that U and V are individually varied while maintaining the ratio U/V at a constant value (for example, see Patent Document 1). For simplicity, the expressions “DC voltage U” and “RF voltage V” will hereinafter be used in place of the aforementioned, exact expressions of U being the voltage value of the DC voltage and V being the amplitude value of the RF voltage.
In a quadrupole mass spectrometer, when a selective ion monitoring (SIM) measurement is performed, the detection of the ions is sequentially conducted for a plurality of predetermined mass-to-charge ratios. In this process, the mass-to-charge ratio being selected by the quadrupole mass filter may be changed by a significant amount. For example, to change the target ion from a low mass-to-charge ratio ML to a high mass-to-charge ratio MH, the set values of the DC voltage U and the RF voltage V must be simultaneously changed by a large amount. During this operation, the voltages actually applied to the rod electrodes do not show an ideal, step-like change; they will inevitably have a certain amount of response time (e.g. rise time, fall time and/or delay time). This poses no problem if both the DC and RF voltages have the same response time and similar transient characteristics. Actually, however, the DC and RF voltages have different response times since they are generated by separate circuits. This situation causes the following problems.
FIGS. 7A-7D are model diagrams for illustrating the problem resulting from the difference in response time between the DC voltage U and the RF voltage V.
When the response time t(U) of the DC voltage U is greater than the response time t(V) of the RF voltage V, the voltage change due to the switching operation between the low mass-to-charge ratio ML and the high mass-to-charge ratio MH will be as shown in FIG. 7A. In this case, as shown in FIG. 7B, a large amount of ions can pass through the quadrupole mass filter in the transient state during the switching operation from the low mass-to-charge ratio ML to the high mass-to-charge ratio MH. Conversely, when the response time t(V) of the RF voltage V is greater than the response time t(U) of the DC voltage U, the voltage change during the switching operation between the low mass-to-charge ratio ML and the high mass-to-charge ratio MH will be as shown in FIG. 7C, where, as shown in FIG. 7D, a large amount of ions can pass through the quadrupole mass filter in the transient state during the switching operation from the high mass-to-charge ratio MH to the low mass-to-charge ratio ML.
This phenomenon is hereinafter explained by using FIGS. 8A and 8B, which show stability diagrams based on the stability conditions for the solution of a Mathieu equation.
The stability region S, in which an ion can exist in a stable state in the quadrupole electric field formed in the space surrounded by the rod electrodes (i.e. in which the ion can pass through the quadrupole mass filter without being dispersed halfway), has a nearly triangular shape as shown in FIGS. 8A and 8B. When the mass-to-charge ratio is changed from ML to MH, the stability region S moves and expands, as shown in FIG. 8A. If the response times t(U) and t(V) are roughly equal (i.e. the voltage ratio U/V is maintained at a substantially constant level), the voltages will change as indicated by the dashed line in FIG. 8A. By contrast, if the change of the DC voltage U is delayed from that of the RF voltage V, the electric field that influences the motion of the ions introduced in the quadrupole mass filter will, in an extreme case, change as indicated by the thick arrowed line in FIG. 8A. In this case, the changing path is largely included in the stability region S, so that ions introduced into the quadrupole mass filter in this transient state can easily pass through this filter without being dispersed.
Conversely, when the mass-to-charge ratio is changed from MH to ML, the stability region S moves and shrinks, as shown in FIG. 8B. In this case, if the change of the RF voltage V is delayed from that of the DC voltage U, the electric field that influences the motion of the ions introduced in the quadrupole mass filter will, in an extreme case, change as indicated by the thick arrowed line in FIG. 8B. In this case, the changing path is largely included in the stability region S, so that ions introduced into the quadrupole mass filter in this transient state can easily pass through this filter without being dispersed.
If an excessive amount of ions pass through the quadrupole mass filter in the transient state due to the switching of the mass-to-charge ratio, an excessive amount of ions will impinge on the detector, promoting the degradation thereof. In the case of a triple quadrupole (tandem) mass spectrometer having front and rear quadrupole mass filters with a collision cell located in between (for example, see Patent Document 2), if an excessive amount of ions pass through the front quadrupole mass filter, an excessive amount of ions will be retained within the collision cell, which may possibly cause crosstalk, deterioration in the S/N ratio or sensitivity, or other problems.